118 HYDRAULIC TURBINES

is of practical importance as showing why impulse wheels have
relatively flat efficiency curves.

80. Illustrative Problem.—Referring to Fig. 93 let the total
fall to the mouth of the nozzle be 1000 ft. Suppose BC =
5000 ft. of 30-in. riveted steel pipe and at C a nozzle be placed
whose coefficient of velocity = 0.97. Suppose the diameter of
the jet from the nozzle == 6 in. Let this jet act upon a tangential
water wheel of the following dimensions: Diameter = 6 ft.,
ai = 12°, 02 = 170°. Assume fc = 0.6, <£ = 0.465, and assume
bearing friction and windage = 3 per cent, of power input to
shaft.

The problem of the pipe line is a matter of elementary
hydraulics and a detailed explanation will not be given of the steps
here employed. The coefficient of loss at B will be taken as

FIG. 93.

1.0, the coefficient of loss in the pipe will be assumed 0.03, The
loss in the nozzle will be given by ( ~~~2 — 11 -y-, where cv = the

coefficient of velocity and Vi the velocity of the jet. If Vc = the
velocity in the pipe then the losses will be

i
Taking HA = 1000 ft. and HI = -75- then by equation (4) we
z#
may solve for-™ = 1.38 ft. or ^ = 861 ft.
zg &g
Thus Vc = 9.42 ft. per second and Vi = 235.5 ft. per second.
Rate of discharge, q = 46.2 cu. ft. per second.
T)
The pressure head at nozzle, ™ = 914.5 ft.
The wheel speed m = 0.465 X 8.025V915:88 = 113 ft. per
second. 4
ThereforeJV = 360r.p.m,